SQL于DML(数据库操作语言)采用

1.Insert语句：

INSERT [INTO] table [(column1, column2, column3, . . .)] VALUES(value1, value2, value3, . . .);
例：INSERT INTO user VALUES(NULL, 'test', '123456');
例：INSERT INTO user (username, password) VALUES('test', '123456');
实际上另一种方式：

例：INSERT INTO user SET username='test', password='123456';

2.查询语句：

SELECT [option] item [INTO file_details] FROM tables [WHERE condition] [GROUP BY group_type] [HAVING where_definition]

[ORDER BY order_type] [LIMIT limit_criteria] [PROCEDURE proc_name(arguments)] [lock_option];

①简单查询

例：SELECT * FROM user WHERE uid=4;

②多表查询

主要的多表查询

例：SELECT user.username, userinfo.age, userinfo.sex, userinfo.phone FROM user, userinfo WHERE user.uid=userinfo.uid;

左关联

SELECT user.uid, uid.name, orders.orderid FROM user LEFT JOIN orders ON user.uid=orders.uid;

在没有使用做关联的情况下仅仅会返回满足条件的记录，假设使用左关联左表将所有返回即使右表并不匹配，所查的右边的值将用NULL取代

（换句话说：左关联将返回全部满足条件的记录。还会将左表没有返回的记录也返回，所需的右表数据如orders.orderid用NULL填充）

③使用子查询

主要的子查询

例：SELECT uid, amount FROM orders WHERE amount=(SELECT max(amount) FROM orders);

关联子查询

例：SELECT isbn, title FROM books WHERE exists (SELECT * FROM orders WHERE orders.isbn=books.isbn); //内部查询引用外部查询数据

行子查询

例：SELECT c1, c2, c3 FROM t1 WHERE (c1, c2, c3) IN (SELECT c1, c2, c3 FROM t2);

使用子查询作为暂时表

例：SELECT * FROM (SELECT uid, username FROM user WHERE city='Beijing') AS user_beijing;

④合计函数与分组

合计函数：

avg()

count()

min()

max()

std()

stddev()

sum()

. . . 

分组通常结合合计函数使用。对结果集进行分组

例：SELECT uid, avg(amount) FROM orders GROUP BY uid;

HAVING类似于WHERE，仅仅用于合计与分组，SQL中添加HAVING的原因是。WHERE中无法使用合计函数

例：SELECT uid, avg(amount) FROM orders GROUP BY uid HAVING avg(amount)>100;

⑤排序与限制的使用

例：SELECT username, password FROM user ORDER BY username ASC;    //查询user表并按username字段升序排序

例：SELECT username, password FROM user ORDER BY uid DESC;    //查询user表并按uid字段降序排序

例：SELECT username, password FROM user LIMIT 4;    //查询user表前四条记录

例：SELECT username, password FROM user LIMIT 3, 4;    //查询user表从第三条開始之后4条记录

3.更新语句：

UPDATE [LOW_PRIORITY] [IGNORE] table SET column=expression1, column2=expression2, . . .

[WHERE condition] [ORDER BY order_criteria] [LIMIT number];

例：UPDATE user SET password='111111';    //将user表中全部人密码设成111111

例：UPDATE user SET password='111111' WHERE uid=4;    //将user表中uid等于4的记录的密码设成111111

例：UPDATE user SET password='111111'    ORDER BY uid DESC LIMIT 5;    //将user表中按uid倒序的前5条记录的密码设成111111

这里的ORDER BY通常与LIMIT结合使用，单独使用ORDER BY没有意义

4.删除语言：

DELETE [LOW_PRIORITY] [QUICK] [IGNORE] FROM table [WHERE condition] [ORDER BY order_cols] [LIMIT number];

例：DELETE FROM user;    //删除user全部数据

例：DELETE FROM user WHERE uid=4;    //删除uid=4的记录

例：DELETE FROM user ORDER BY uid DESC LIMIT 5;    //删除user表中按uid倒序的前5条数据

提示：[]表示可选

           在字段名上加上反引號``能够避免字段名与数据库keyword冲突

           弦值需要使用引号''行情，号码值不需要

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